1. Field of the Invention
The present invention relates to the field of creating smooth surfaces on sheet materials, and more particularly to an apparatus for controlling the cross-directional smoothness profile of a paper sheet.
2. Related Art.
One of the parameters used in grading sheet materials is the smoothness of the material's surface. In the paper production process, various grades of paper having different surface smoothness are produced to suit various applications. Generally, smooth surfaces enhance the printability of the paper. Bulk paper is typically produced in a continuous sheet and wound in rolls having dimensions 12-36 feet in the cross-direction (i.e., across the width of the sheet) and uniform smoothness on the paper surface is generally desirable. For example, in the situation where the roll of paper is cut into page-size sheets, the consistency of the smoothness of the individual pages is dependent upon the uniformity of the smoothness of the original bulk paper roll.
Paper production typically involves a calendering process which includes pressing paper material between two or more calender rolls arranged in a stack, to obtain desired physical characteristics. Calendering paper can change its density, thickness (caliper), and surface characteristics, including smoothness. In conjunction with calendering, steam is frequently applied to paper before it is calendered so as to moisten and heat the paper and thereby affect certain of its characteristics. For example, both the caliper of the paper and the smoothness of its surface may be impacted by applying steam to the paper surface, followed by pressing the paper between a series of calender rolls. The paper absorbs the steam and the paper fibers are softened by the heat and moisture thereby increasing the pliability and compressibility of the paper. As the steam-treated paper comes into contact with the calender rolls, it is then compressed and its surface is smoothed by the "ironing" (i.e., pressing and rubbing) actions of the rolls. The caliper and smoothness profiles created are dependent on the amount of moisture and heat penetrating the paper. Typically, to achieve the desired smoothness of the paper surface, only the surface fibers of the paper need to be wetted and heated. To substantially affect the caliper profile of the paper, on the other hand, the steam must be allowed to penetrate deeper into the paper.
A common problem encountered in using a steam treatment prior to calendering paper to affect the smoothness of the calendered material, is the concurrent effect on the caliper of the material. "Coupled" to the increase in the smoothness of the paper is a decrease in its caliper. More predictable caliper and smoothness profiles of paper could be achieved if the two characteristics could be "decoupled" (i.e., controlled independently) by applying steam so as to heat and wet the surface fibers only after the desired caliper profile has been created.
Another common problem encountered in affecting the smoothness of the calendered material using a steam treatment is the non-uniformity of the smoothness achieved in the cross-direction. Localized variations in the amount of steam applied to the surface of the bulk paper may affect the smoothness uniformity. Also, there are other variables in the calendering process such as temperature and calender roll pressure which may affect the amount of steam required for a particular degree of smoothness. A more uniform smoothness profile can be obtained if the amount of steam directed at different sections of the paper surface can be controlled.
A further problem associated with the application of steam in calendering is that excess steam that has not been absorbed by the paper condenses on cool surfaces of the adjacent structure of the calender system. For example, the steam may condense on the calender roll, which will wet the paper as the roll contacts the paper. The extra moisture of the calender roll in addition to the moisture applied directly to the sheet from the steam supply will affect the moisture distribution and hence the smoothness and other physical properties of the paper. For example, when droplets of water contact the sheet and the sheet is subsequently calendered, the opacity of the sheet will be permanently affected in the wetted area, thereby leaving a visible mark on the sheet. In addition, excess steam may condense on a cool portion of the paper surface at a location where steam treatment is not intended, thereby affecting the smoothness profile.